Table of Physical and Chemical Constants

Author: G.W.C. Kaye / et al
Publisher:
Publish Date: 2003-01-01
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Fragment: 1.1.1 The International System of Units (SI) History
In the second half of the nineteenth century, the centimeter, gram, and second were in fairly general use as base units for scientific work even in such countries as the UK and the USA where the foot and the pound were employed for commerce and engineering. As a result, the units required by the rapidly emerging science of electricity were based on the centimeter, gram, and second, with which they formed a coherent system known as the CGS electromagnetic system. A system of units is said to be coherent when derived units are formed from the base units without the insertion of factors of proportionality other than unity. There was also the CGS electrostatic system, but the only quantities frequently expressed in electrostatic units were electric charge, electric potential, and capacitance. The young but fast-growing electrical industry soon found that many CGS electromagnetic units were of an extremely inconvenient size for its needs. Accordingly, in 1881, international agreement was reached to fix the practical unit of potential, to be called the volt, at 108 CGS units (which is approximately equal to the e.m.f. of a primary cell), and the unit of resistance, the ohm, at 109 CGS units (which is approximately the resistance of a column of mercury 1 ml long and 1 mm2 in cross-section). The unit of electric current, the ampere, was made a tenth of the CGS unit. A coherent system of practical electric units was thus secured which, however, was not coherent with the mechanical units based on the centimeter and gram. The practical electric units suited the needs of telegraphy, which was then the main electrical industry, and they also happened to be convenient for heavy electrical engineering and for electronics. The magnetic units, however, were left at their CGS values, presumably because the CGS unit of magnetic flux density, subsequently called 'gauss', is of the order of the flux density of the Earth's field, and as it was suitable for ge magnetism, there seemed no point in changing it for a unit 104 times larger. Coherence was thereby lost to electromagnetism as it had already been lost to the system embracing the mechanical units and the practical electric units. Whereas the electric units, by the agreement of 1881, were chosen to be of suitable magnitude for everyday use, and whereas the centimeter and the second have acceptable sizes, the gram is too small for the practical needs of man, which are better served by a unit nearer the size of the pound or the kilogram. Moreover, the CGS unit of force, the dyne, and the unit of energy, the erg, are much too small. On the other hand, the unit of energy provided by the practical electric units, the volt-ampere-second, called the joule (which equals 107 ergs) -- is of a satisfactory size. These considerations -- the advantages of coherence and the fortuitous circumstance that a mechanical system based on the meter and the kilogram has precisely the same unit of energy as is provided by the practical electric units -- led G. Giorgi in 1902 to propose a system based on the meter, the kilogram, the second, and one of the practical electric units. He pointed out that if magnetic field strength were expressed as amperes per meter instead of 4π times amperes per meter, which is the definition corresponding to that of the CGS unit, then the number π would disappear from most electric and magnetic formulae involving rectilinear geometry, but would appear, as is to be expected, in those involving cylinders or spheres. The International Electrotechnical Commission eventually chose the ampere as the fourth base unit of the MKSA or 'Giorgi's system', and in 1948 the 9th General Conference of Weights and Measures recommended it for science and technology, as well as for commerce and industry. This system admirably